The A domain of fibronectin-binding protein B of Staphylococcus aureus contains a novel fibronectin binding site.

Abstract

The fibronectin‐binding proteins FnBPA and FnBPB are multifunctional adhesins than can also bind to fibrinogen and elastin. In this study, the N2N3 subdomains of region A of FnBPB were shown to bind fibrinogen with a similar affinity to those of FnBPA (2 μm ). The binding site for FnBPB in fibrinogen was localized to the C‐terminus of the γ‐chain. Like clumping factor A, region A of FnBPB bound to the γ‐chain of fibrinogen in a Ca2+‐inhibitable manner. The deletion of 17 residues from the C‐terminus of domain N3 and the substitution of two residues in equivalent positions for crucial residues for fibrinogen binding in clumping factor A and FnBPA eliminated fibrinogen binding by FnBPB. This indicates that FnBPB binds fibrinogen by the dock–lock–latch mechanism. In contrast, the A domain of FnBPB bound fibronectin with K D = 2.5 μm despite lacking any of the known fibronectin‐binding tandem repeats. A truncate lacking the C‐terminal 17 residues (latching peptide) bound fibronectin with the same affinity, suggesting that the FnBPB A domain binds fibronectin by a novel mechanism. The substitution of the two residues required for fibrinogen binding also resulted in a loss of fibronectin binding. This, combined with the observation that purified subdomain N3 bound fibronectin with a measurable, but reduced, K D of 20 μm , indicates that the type I modules of fibronectin bind to both the N2 and N3 subdomains. The fibronectin‐binding ability of the FnBPB A domain was also functional when the protein was expressed on and anchored to the surface of staphylococcal cells, showing that it is not an artifact of recombinant protein expression.